Recent development and distribution of mobile tools and electric motors have led into increased demand for high-capacity energy sources such as lithium secondary batteries, for representative example. At present, carbon material such as graphite, hard carbon, etc. is used for the negative electrode active material for the lithium secondary battery.
For example, graphite has a theoretical capacity of 372 mAh/g defined by the first stage structure C6Li formed by intercalation reaction, and it has been used in increasing amount to achieve higher capacity of the battery so far, but now reached the limit. Further, hard carbon may be used and may achieve capacity exceeding the theoretical capacity of graphite, but it can hardly realize high-capacity secondary battery, considering factors such as low initial efficiency, low electrode density, and so on. Considering the above, use of silicon for a negative electrode active material has been proposed, since silicon, as a novel material that can replace the carbon material, exhibits theoretical capacity as high as 4200 mAh/g by alloying with lithium.
Japanese Unexamined Patent Application Publication No. Hei 6-325765 (Patent Document 1) proposes lithium ion occluding and releasing material consisting of Li-containing silicon oxide or silicate. Use of silicon oxide mixed in a small amount with graphite has been commercialized, since the coexistence of amorphous or microcrystalline silicon phase and silicon oxide in single particles can provide relatively desired charge-discharge cycle characteristics. However, since silicon oxide has 20% or lower initial efficiency than graphite, if used in an increased mixing amount, it will disrupt initial efficiency balance with the positive electrode. Further, it becomes practically difficult to increase energy density.
Further, general understanding is that when silicon as well as silicon oxide is used as a negative electrode active material, silicon is pulverized due to repeated expansion and contraction during charging and discharging, thus resulting in generation of gaps within the electrode, which cut off conduction pathways and cause increased amount of silicon that does not contribute to charging and discharging and reduced battery capacity and deteriorated cycle characteristic.
Japanese Laid-Open Patent Application Publication No. 2004-103340 (Patent Document 2) proposes a solution to the issue associated with the use of high-capacity silicon-based material as the negative electrode active material, by forming expansion inhibiting phase by way of alloying Li-occluding metal such as silicon, tin, zinc, etc. with a Group 2A element or a transition metal, and also by microcrystallizing Li-occluding metal, thereby inhibiting cycle deterioration due to charging and discharging.
Further, Japan Laid-Open Patent Application No. 2008-023524 (Patent Document 3) proposes a negative electrode material for lithium ion secondary battery comprising a composite material which is treated to be imparted with compressive and shear forces and which has a structure in which silicon particles having carbonaceous film on at least a portion of the surfaces are in close contact with graphite material. According to Patent Document 3, close adhesion among metal particles and among metal particles and carbonaceous material prevents separation of the meta particles from one another, and separation of the metal particles from the carbonaceous material due to expansion and contraction that accompany charging and discharging. Accordingly, discharge capacity is higher than theoretical capacity (372 mAh/g) of graphite, and is considered to be the one that can provide a negative electrode material for lithium ion secondary battery having desired cycle characteristic and initial charge and discharge efficiency.
However, the suggestion of Patent Document 2 still suffers inefficient charging and discharging, because the surface of the alloy particles are apt to oxidize and has lower conductivity compared with carbonaceous material. Particularly at the end of discharging when electric resistance of the active material increases, the insufficient discharge causes retention of lithium ion inside near the surface of the particles, thus causing degradation of the battery. Further, when the binary or ternary elements alloying with Li during lithium ion intercalation and de-intercalation has metal element such as Al, and so on, there is a presence of a plurality of Li occlusion phases including silicon, and as a result, degradation is accelerated.
Further, regarding the suggestion of Patent Document 3, when crystalline silicon having considerably high expansion ratio is composited with graphite, due to 4-fold or greater difference of expansion ratio between the two, the expanded silicon falls, rather than staying only within the pores in the graphite, and thus the particle decay occurs. As a result, cycle deterioration can occur.